Metal container



No'v.-26 1940. P. M. CLARK ETAL 2,223,026

' METAL CONTAINER Filed Nov. 20, 1957 TUNMODIFI D NIT ROCELLULO. E FILVIBu'fyl Oleafe #www yl Pwfhaiafe GRAMS 0F WATER PASSED PER SQUARE METERPER HOUR PERCENT PLASTICIZER Percy M. C Zar/f I N VEN T0125 Char/e:Henwnmg Patented Nov. 26, 1940 UNITED STATES PATENT OFFICE METALCONTAHNER Delaware Application November 20, 1937, Serial No. 175,627 I 3Claims.

This invention relates to a metal container for the packaging of foodproducts or other perishable materials and more particularly to theproduction of an interiorly coated metal U container, the coating ofwhich is capable of protecting the metal from corrosion due to corrosiveagents present in the packed material.

Metal containers interiorly coated with organic coatings of theoleoresinous type have been l used for many years in the packaging ofvarious foods, especially Where the acid nature of the materials tendsto attack the metal container and subsequently cause contamination ofthe food material through the absorption of the l5 products formed bythe corrosive action. It has been found in general* that various classesof foodstuffs demand various types of protective coatings and much workhas been done by the manufacturers of tin cans to provide coatings foreach specific use.

A more recent development in the can industr'y has been the productionof cans for the packaging of beer. It was found that the usual coatingswere unsatisfactory and incapable of delivering the beer in a conditionsuitable for public consumption. Several factors appeared to cause thiscondition, among them, absorption of flavor-forming ingredients by thecan coating, and the presence in the coating of soluble ingredientswhich when absorbed by the beer, markedly changed the taste thereof. Inparticular, it has been found that beer is especially subject tocontamination land deterioration. Two types of coated containers havebeen developed which are being used commercially in large numbers at thepresent-time. The first is a metal can with or without an organic primercoating and having a heavy wax coating as the top coat. The second typeis a container with an organic priming coat and having a vinyl resin topcoating.

Prior to and during the development of the above types of cans,considerable work was done by the industry in attempting to developnitrocellulose lacquers for use as top coats and that these results wereuniformly unsuccessful. Several reasons can be ascribed to thesefailures. First, failure to select suitable materials with which tomodify the nitrocellulose. Second, failure to study the bakingconditions and the solvent compositions required to give coatings freefrom traces of residual solvent which if present would cause seriouscontamination of the beer. Third, failure to Work with modifierspurified to the extent absolutely necessary for this work, and finallyand more importantly, failure to recognize the necessity of reducing thepermeability of the nitrocellulose film coating to Water to a point muchlower than commonly reached. Unmodifed nitrocellulose coatings are muchmore permeable to water than is generally recognized, and when notsuitably modified are inferior to the vinyl resin and Wax coatings incommercial use.

The presentfinvention is unique and commercially of interest in that theseveral defects of the nitrocellulose compositions and coating methodsheretofore available for lining the interior of metal cans utilized inthe packaging of beer, food products or other perishable materials; areeliminated. An object of this invention is'the provision of aninteriorly coated container for the packaging of beer and the like, thecoating of which is Water-resistant, tough, flexible, adherent,insoluble, non-toxic, substantially impermeable to water and capable ofprotecting the metal from corrosion due to corrosive agents present inthe packed material. Other objects will become apparent as thedescription of the invention proceeds.

These objects are accomplished by utilizing a specic type of modifier,consisting of a cyclohexyl derivative, in cellulose nitratecompositions, thus affording a lacquer which when 0 coated on theinterior of metal cans provides a convenient and commercially usefulcontainer.

In the practice of the invention, a priming coat is first applied to themetal surface before the application of the lacquer top coat. Thequalifications for the primer are freedom from taste-contamination afterbaking, good adhesion to the metal surface and satisfactory coverage.The primer may be applied to flat sheets of black iron or tin orsilver-plated metal by a roller coating machine at a prescribed coatingweight which normally is about 4 milligrams of dry coating per squareinch. The primer is baked at a temperature of l about380425 F. for to 15minutes. Sections of the flat sheets are then formed into cylinders andthe top coat applied to the interior .by any suitable means, preferablyby spraying. The can ends are usually coated with the lacquer beforebeing formed. However, it is practicable to form the can ends directlyfollowing the baking of the primer and before the application of thelacquer top coat. In this case the lacquer would normally be applied byspraying.

whichy isa reaction product of zvExample 1 A suitable primer compositionconsists of, an" mixture of.5.5 parts of mineral spirits, 61.5 parts ofhydrogenated naphtha of boiling range about 135 "to 190 C., and 33'partsof a synthetic resin APer cent by weight Chinawood oil 12.3, Glycerine"-18.8 Linseed oil 24.7 Phthalic anhydride 44.2

This resin is prepared by any of the methods well known and widelypracticed in the art.

An alternative priming composition may consist of an oleoresinousvarnish of conventional type, being prepared in the customary mannerfrom drying oils .and natural or synthetic gums. In general, it has beenshown -that oil lengths above gallons in this type of base coat areliable to give a pronounced taste-contamination and the selection of theraw material for its preparation should be made with a view tominimizing its taste-producing characteristics.

`Other commercially available priming compositions of similarconstitution and properties may also be utilized as suitable base coatsfor the improved lacquer top coats hereinafter described.

The lacquerA top `coat is applied byspraying, roller-coating or dipping.It is then baked at a temperature not to exceed 300 F. for 8 to 10minutes. The following examples are illustrative as representing thepreferred embodiment of this invention:

Example 2 Y Per cent by weight Cellulose nitrate 14.6

Dicyclohexyl phthalate 4.4

Ethyl alcohol (denatured) 3.7

' Ethyl acetate (absolute) 77.3

Example 3 Per cent l by 'weight Cellulose nitrate' 14.6 Dicyclohexyl.hexahydrophthalate 7.3

Ethyl alcohol (denatured),. 3.1

Ethyl acetate (absolute) 50.0

Isopropyl acetate l 24.4

Example 4 Per cent by weight Cellulose nitrate 15.0

Dimethyl cyclohexylphthalate 5.8

Ethyl alcohol (denatured) 4.0

Ethyl acetate 30.0

Methyl ethyl ketone 30.0

Isobutyl acetate 15.2

The viscosity characteristic of the cellulose nitrate used in the aboveexamples is about 3 seconds as determined in accordance with procedureoutlined in A. S. T. M. specifications D-30l-33, Formula B. 'Use of alower viscosity material is possible but in general it leads to somewhatbrittle films which do not permit satisfactory forming of the coatedsheet metal. Use

.of higher viscosity cellulosenitrate while preferable from theviewpoint of giving tougher lms resultsin av lacquer having very lowsolids at the required spraying or applying viscosity.` It-is desirableto use the lowest viscosity cellulose ni'- trate commensurate with theflexibility desired o1V the iilmv in order to produce the mosteconomical shes. l v y In Exam le 2, absolute ethyl acetate is used asthey sole volatile solvent forthe cellulose nitrate. The small amount ofdenatured ethyl alcohol is brought into thecomposition by the alcoholwet cellulose nitrate of commerce. This represents a volatile solventvehicle which has been found to be the easiest to completely eliminatein the baking operation of the lacquer top coat.V It is possible toemploy a variety of other solvent 'combinations providing satisfactorybaking conditions are set up. Medium boiling solvents such as isopropylacetate, isobutyl acetate,

normal butyl acetate, and in some instances volbaking treatment willinvariably contaminate the packed product.

Highly purified, odorless and tasteless dicyclohexyl phthalate was foundto be outstandingly superior as a plasticizer in the cellulose nitrate.compositions for the purpose indicated. However, dicyclohexylhexahydrophthalate, dimethyl cyclohexyl phthalate, dicyclohexyltartrate, tricyclohexyl citrate,` cyclohexyl benzoyl benzoate andtrlcyclohexyl phosphate, and other cyclohexanol derivatives ofcarboxylic acids, when subjected to suitable puriiication treatments torender them odorless and tasteless are likewise suitable for use in theproducts of the present invention. v

In addition to the normal function of the plasticizer which is theflexibillzing of the cellulose nitrate coating, this class ofplasticizing materials greatly reduces the permeability of the lacquer'lm to the passage of Water. The great majority of plasticizers of whichdibutyl" phthalate, castor oil and butyl oleate are representative, whenadded to nitrocellulose in at least up to about equal parts by weight fcel-r` lulose nitrate and plasticizer.

The superiority of the compositions of the present invention as regardsliquid water permeability is clearly illustrated in the attached seriesof curves. The curves are derived by plotting the liquid waterpemeability of iilms prepared from the severaly compositions againstplasticizer content and indicate the improved permeability factor whichis obtained as the cyclohexanol derivative plasticizer content isincreased to the desired range. .The relatively poor permeabilitycharacteristic of compositions containing dibutyl phthalate and butyloleate is also i1- lustrated for comparison.

Numerical data from which the curves were increasing amounts at iirstcause a sharp lowering* in perplotted was obtained as follows: Acomposition such as Example 2 is coated on both sides of a sheet ofplain, transparent regenerated cellulose such as Cellophane to anoverall thickness of .0004 inch of dry film. The coating is dried in adry oven at 180-200 F. for one minute with circulating air. The driedfilm is then afxed across the top of a Petri dish, approximately 21/2inches in diameter by means of a molten adhesive composed of beeswax 85%and damar 15%, a quantity of water sufficient to completely cover thecoated Cellophane when the dish is inverted, having been previouslyplaced in the dish. The dish is accurately weighed, inverted and placedon a rack arranged so as to allow free circulation of air across the lm.It is then placed in a thermostatically controlled oven at a temperatureof 104 F. in which a relative humidity of less than 2% is maintainedwith forced air circulation, the air entering the oven having been driedby passing it through a series of bottles containing concentratedsulfuric acid. At the end of 18 hours the dish is removed from the ovenand again accurately weighed. The results are expressed as grams ofwater lost per square meter per hour as utilized in plotting the curveson the accompanying graphs.

Minimum permeability is obtained when the cellulose nitrate-plasticizerratios are within the range of 10-3 to 10-5 parts by weight. Asatisfactory permeability reduction occurs with this new type of lacquercoat within the range of 10-3 to 10--10 parts by weight of cellulosenitrate to plasticizer.

In all instances this type of modifier greatly improves the adhesion andflexibility of the cellulose nitrate iilm and aids in the elimination ofthe last traces of solvent from the lacquer coatingl It is tasteless,odorless, non-toxic and inert to hydrolysis.

This invention provides an interiorly coated metal container which iscommercially satisfactory for the packaging of beverags, particularlymalt beverages, food products and other perishable materials. The newcontainer meets the need in the industry for a can in which beer may besatisfactorily packed. It provides an interior surface coating which isporeless, substantially imperviousto water, odorless and tasteless andcapable of withstanding the deteriorating effect of beer and otherbeverages such as fruit juices and perishable foodstuffs which may alsobe safely packaged in the new containers.

It is apparent that many Widely different em-A bodiments of thisinvention may be made without departing from the spirit and scopethereof and, therefore, it is not intended to be limited except asindicated in the appended claims.

We claim:

1. A metal container for beer and other beverages, foodstuffs and thelike consisting of a tin plated metallic body interiorly coated with analkyd resin priming coat and a top vcoat consisting of cellulose nitrateand a plasticizer compatible therewith, said plasticizer consisting of acyclohexyl ester of acarboxylic acid in the ratio of between l parts byweight of cellulose nitrate to l0 parts of said plasticizer and 10 partsby weight of cellulose nitrate and 3 parts of said plasticizer.

2. A metalcontainer for foodstuffs containing water, having a. coatingon at least the surface of the said metal container which is in contactwith the foodstuff, said coating comprising in proportion parts ofnitrocellulose and from 3 to 10 parts of a highly puried cyclohexylester of a carboxylic acid as a plasticizer.

' 3. The container of claim 2 in which the plasticizer is selected fromthe group consisting of cyclohexyl and methyl cyclohexyl esters of themono and dibasic carboxylic acids.

PERCY' M. CLARK. CHARLES B. HEMMING.

